Tertiary Uplift and Erosion in the Barents Sea: Magnitude, Timing and Consequences

Abstract

Abstract In 1920 Fridtjof Nansen postulated an uplift and erosion in the order of 500 m in the Barents Sea. Hydrocarbon exploration in the 1980's has since confirmed a significant uplift and erosion in this area. In 1985 Statoil initiated a multidisciplinary study to evaluate and quantify the amount of uplift and erosion. The methods used were: Shale compaction curves, Dc-exponent from drilling, vitrinite reflectance trends, diagenesis of clay minerals, apatite fission track studies, volumetric mass balance calculations of erosion and deposition, and PVT-modelling of known reservoirs. Studies of porosity and density trends in Tertiary shales from wells showed an uplift and erosion of the order of 500–1000 m in the Hammerfest Basin, increasing to about 1700–1800 m towards the Fingerdjupet Subbasin area. Vitrinite trends from exploration wells and IKU shallow drillings confirmed these results also showing increasing erosion towards the north (3–4 km erosion at Bjornoya). Depending on the paleo-drainage area chosen, volumetric calculation of the Late Tertiary deposition along the western shelf margin gives a magnitude of erosion of about 1–1.2 km in the southern part of the Barents Sea, increasing to approximately 3 km on Svalbard. Used together, the methods above give a fairly accurate estimate on the magnitude of the erosion but not on timing. Fission track studies indicate one or more phases of uplift from Late Oligocene to Pliocene/Pleistocene, while biostratigraphic redatings of sediments from the wells on the Senja Ridge suggest that a major part of the erosion is related to glacial processes during the last 2–3 Myr. Extensive residual oil staining found below the present hydrocarbon/water contacts indicates that reservoirs, such as the Snohvit field in the Hammerfest Basin, once contained larger quantities of oil. PVT-modelling has shown that this oil was lost probably as a consequence of gas expansion during uplift, and can explain in part the lack of commercial oil finds so far in the Barents Sea.